The present invention relates to an electro-hydraulic servomotor used for hydraulic shovels, cranes, asphalt finishers and machine tools (those machines will be referred to simply as external machines).
In this type of the electro-hydraulic servomotor, as shown in FIGS. 13 and 14, an output shaft 2 is rotatably supported on a casing 1 by bearings 3 and 4. A valve plate 9 is fastened to the inner wall of the casing 1, and a cylinder block 7 is fastened to the circumferential portion of the output shaft 2. A plurality of pressure chambers 7a is formed in the cylinder block 7. Pistons 8 are disposed within those pressure chambers 7a, and the pistons 8 are reciprocally moved in their axial direction by a hydraulic pressure of an operation oil introduced into the pistons 8.
A slanted plate, which is slanted at a given angle with respect to the valve plate 9, is fastened to a portion of the inner wall of the casing 1 which is closer to the top end of the output shaft 2. The top ends of the pistons 8 slidably push the slanted plate 6, and the cylinder block 7 slides to the valve plate 9, whereby the output shaft 2 and the cylinder block 7 are rotated together.
A spool valve 11, which moves in the axial direction, is provided in the casing 1. A screw member 12 and a gear 13 are fastened to the top end and the base end of the spool valve 11, respectively. A pulse motor 14 is mounted on the casing 1. A motor shaft 15 of he pulse motor 14 is rotatably supported on the casing 1. A rotational force of the motor shaft 15 is transmitted to the spool valve 11 via gears 16 and 13. A rotational force of the output shaft 2 is transmitted to the spool valve 11 via screw members 10 and 12. When the spool valve 11 is turned, an oil discharging passage 1, an oil supplying passage 1b, and communicating passages 1d and 1d communicate with one another. In the electro-hydraulic servomotor, the output shaft 2, the spool valve 11 and the pulse motor 14 are disposed on the same axial line.
Since in the thus constructed electro-hydraulic servomotor, the output shaft 2, spool valve 11 and the pulse motor 14 are disposed on the same axial line, the entire length of it is long. For this reason, it is difficult to neatly assemble the electro-hydraulic servomotor into another machine. A speed ratio of the screw members 10 and 12 is 1:1. Because of this, to increase the spindle speed of the output shaft 2, it is necessary to increase a capacity of the pulse motor 14 and to drive the pulse motor 14 at high speed. The spool valve 11 rotates together with the screw member 12. Therefore, a sliding surface of the casing 1, which is in contact with the spool valve 11, will be worn because of presence of its friction resistance.
Accordingly, an object of the present invention is to provide an electro-hydraulic servomotor which is small in size.
Another object of the present invention is to provide an electro-hydraulic servomotor which enables the capacity of it to be reduced, and is free from wearing of the spool valve and the casing.
Another object of the invention is to provide a small electro-hydraulic servomotor which reliably controls a spool position of the spool in the axial line direction independently of temperature of the operation oil.
As a preferred embodiment of the present invention, an electro-hydraulic servomotor is provided, which includes: an electric motor which rotates a drive shaft in response to an inputted signal; a hydraulic motor which rotates an output shaft using hydraulic pressure of operation oil; a first geared shaft rotatable along with the output shaft; a second geared shaft threadingly engaged with the drive shaft and meshed with the first geared shaft; a spool axially movable along with the second geared shaft depending on a rotational difference between the drive shaft and the first geared shaft to control supply and discharge of the operation oil to and from the hydraulic motor. According to the servomotor can be made small in size.
In the electro-hydraulic servomotor, the spool may be constructed as a single integral member, maybe divided into first and second discrete spool members. The first and second spool members are preferably urged toward one another.
The electro-hydraulic servomotor may further include: a displacement sensor which detects an axial position of the spool.
The electro-hydraulic servomotor may further include: a rotary sensor which detects number of rotation of the first geared shaft.
The present disclosure relates to the subject matter contained in Japanese patent application Nos. Hei. 11-13633 (filed on Jan. 21, 1999), Hei. 11-291477 (filed on Oct. 13, 1999), Hei. 11-291478 (filed on Oct. 13, 1999) and Hei. 11-348927 (filed on Dec. 8, 1999), which are expressly incorporated herein by reference in their entireties.